Background and Objective: These studies are targeted towards understanding the pathogenesis of psoriatic arthritis and the development of specific immunotherapies for psoriatic arthritis, a relatively common chronic inflammatory disease affecting the skin and joints. Patients with known or suspected psoriatic arthritis are evaluated at the Clinical Center. Studies include characterization of the clinical and laboratory features of the disease, research studies of the natural history, pathogenesis and determination of the patients' eligibility to enter experimental therapeutic protocols. Results: Recruitment in this study was slow over the last year primarily because the beneficial effect and wide availability of new TNF-inhibitors in the community. Lay Summary: Psoriasis is a common inflammatory disease of the skin that affects approximately 2% of the population of the USA. Psoriasis is accompanied by a destructive arthritis in 5-15% of patients. Our studies examine the relative contribution of certain types of inflammatory cells and their products in causing this disease. Thus far we have demonstrated that certain inflammatory cells called T lymphocytes and monocytes, are present in the inflamed joints and are important contributors of the inflammatory process. In addition, we have identified several molecules produced by these cells that cause inflammation and could be targets for future therapy.